Heretofore, tripping of cricuit breakers equipped with static trip units has been predicated on detection of the peak amplitude of any overcurrent flowing in the power distribution circuit being protected, as sensed by iron-core current transformers individually linked with the load current carrying conductors. Unfortunately, for high level fault currents, typical iron-core current transformers utilized in static trip circuit breakers saturate well before the fault current achieves its peak. Once the current transformers go into saturation, information as to the instantaneous amplitude of the fault currents disappears since the secondary voltage simply collapses to zero. It would be desirable, from the standpoint of nuisance trip prevention, to set the instantaneous trip for a particular circuit breaker frame size at an overcurrent level just below its so-called "popping" level, i.e., the level of overcurrent at which the breaker contacts will be forced apart by the extreme electromagnetic forces associated therewith. This popping level may be, for example, as high as thirty-five times the breaker's continuous current rating. Yet, typical iron-core current transformers utilized in such circuit breakers will be driven into saturation at much lower overcurrent levels, e.g., fifteen times the breaker's continuous current rating. Under these circumstances, it is typically not possible to take advantage of the full short-time current rating of a particular circuit breaker design which is defined as the time duration that a particular overcurrent level can be endured before the breaker contacts pop open.
Moreover, pure overcurrent peak detection is not completely satisfactory from the standpoint of nuisance tripping since undesired instantaneous breaker tripping can result from "offset" current transients (exponentially inherent in switching inductive loads and decaying transient components) usually encountered in high level faults.
In order to raise the saturation level of iron-core current transformers, the size of the core must be increased. The current transformers then become so large as not to be readily accommodatable in molded case circuit breakers. The alternative is to install the current transformers externally of the breaker case which takes up additional switchboard space, adds installation expense, and, due to the necessary external wiring, degrades reliability.
Several solutions to this problem have been disclosed in the patented prior art. In U.S. Pat. No. 3,846,675, it is proposed to predicate circuit breaker tripping on sensing how soon after a current zero an iron-core current transformer is driven into saturation by a sinusoidal high fault current wave. By this approach, the eventual peak amplitude of the current wave is anticipated. An alternative approach disclosed in this patent is to utilize a non-saturable, air-core current transformer to develop a full transformation of the fault current wave in its secondary windings which is then peak detected pursuant to deciding whether or not to initiate a circuit breaker trip function. Still another approach is disclosed in U.S. Pat. No. 3,673,455, wherein the secondary winding of a current transformer inductively coupled with a current carrying conductor is connected in series circuit with the primary windings of an iron-core current transformer and an air-gap-containing iron-core current transformer. The iron-core current transformer secondary is adapted to develop a first voltage signal proportional to the instantaneous magnitude of the current flowing in the conductor, while the secondary of the air-gap-containing current transformer is adapted to provide a second voltage signal proportional to the rate of change of the current flowing in the conductor. The two voltage signals are separately level detected, and if they both exceed respectively pre-selected threshold levels, a circuit breaker trip function is initiated.
It is accordingly an object of the present invention to provide an improved instantaneous trip mode network for static trip circuit breakers.
Another object is to provide an instantaneous trip mode network of the above character which predicates circuit breaker tripping solely on the rate of change of the currents flowing in a power distribution circuit being protected.
A further object is to provide an instantaneous trip mode network of the above character which utilizes non-saturable current transformers for sensing the currents flowing in the power circuit conductors.
Yet another object is to provide an instantaneous trip mode network of the above character which utilizes air-core current transformers capable of developing secondary voltage signals directly proportional to the rates of change of the currents flowing in the power circuit conductors.
A still further object is to provide an instantaneous trip mode network of the above character which is less prone to initiating nuisance circuit breaker tripping.
A further object is to provide an instantaneous trip mode network of the above character which is inexpensive to implement and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.